Thermal Interface Materials for Flip Chip BGA

CASE STUDY

Abstract
Increased functionality and performance requirements for microprocessors and ASICs have resulted in a trend to package these devices in the flip-chip BGA form factor (FCBGA). Because these devices use in excess of 40-100 Watts of power, their packages must dissipate heat in an extremely efficient manner. Most semiconductor companies have developed some type of thermally enhanced FCBGA package that provides heat dissipation through the back of the die to a heat spreader. This design works very well, but is highly dependent on the ability of the thermal interface material (TIM) to transfer thermal energy efficiently from the back of the die to the heat spreader. The TIM is often the limiting factor for heat dissipation, either because it does not conduct heat adequately, or because it fails to maintain intimate contact between the back side of the die and the heat spreader. A family of advanced thermal interface materials for high power FCBGA packages is discussed. These silver-filled adhesives provide for high reliability on laminate FCBGA packages. Laser flash thermal testing is utilized to demonstrate that these materials not only have low bulk thermal resistance, but also very low interfacial, or contact resistance. These adhesives have a low modulus and high adhesion, which enables them to flex and remain bonded as the laminate package undergoes stress during temperature cycling. Data also show that this family of TIMs has very low moisture absorption, which contributes to excellent adhesive reliability during HAST (highly accelerated stress test). Reliability data on laminate test packages will be presented. This family of materials has been shown to pass more than 1000 cycles of temperature cycling B (-55 to +125°C, liquidto-liquid) and 200 hours HAST (121°C, 100% relative humidity), after JEDEC Level 3 preconditioning and three reflow simulations at 220°C on a laminate package with 10 x 10 mm die. Total thermal resistance of less than 0.10 cm2K/W has been achieved, at a 25μm bondline thickness. Thermal interface materials must continuously improve to
keep pace with ASIC and microprocessor technology. The development of these novel thermal interface materials will enable the next level of performance, both in terms of thermal dissipation and package reliability.

Introduction
The demands of the electronics industry have pushed electronic assemblies to operate at increased speeds and in smaller form factors with higher reliability. These industry requirements create thermal management challenges for the electronic assembly’s semiconductor components. In particular, the recent development of thermally enhanced flip chip BGA packages for high I/O ASIC and microprocessor devices has driven semiconductor adhesive suppliers to develop new thermal interface materials to support this package design. Thermal interface materials (TIMs) including conductive pastes, greases, phase change materials (PCMs), thermal pads and films are used as the interface between the flipped IC and an integrated heat spreader.

Figure 1. A simplified presentation of Flip-Chip Ball Grid Array.

Figure 1 shows a typical cross-section of a thermally enhanced FCBGA that uses a conductive paste TIM. The flipped IC is attached to a laminate substrate via solder balls and is underfilled with an encapsulant material. A heat spreader is attached to the back of the die via a thermally conductive material, the TIM. The heat spreader is then attached to the BGA board with a lid seal adhesive. System level thermal solutions are complex, with a number of factors contributing to their success. The designer must select the proper thermal compounds, heat spreaders, heat sinks, fin designs, and device location. At the package level, the thermal interface material employed in a thermally enhanced flip chip BGA must provide four main functionalities:
1. Sufficiently low thermal resistance to maintain the IC operating temperature in its target range,
2. A low stress bond between the coefficient of thermal expansion (CTE) mismatched silicon die and the (typically copper-based) heat-spreader,
3. Good adhesion and consistent thermal performance after standard JEDEC preconditioning, temperature cycling, and moisture resistance testing, and
4. Good compatibility with current adhesive dispensing equipment processes.

Low Thermal Resistance
The thermal requirements of FCBGAs depend upon the amount of power dissipated by the IC during operation. While ASICs today may range from 2-10 watts of power, microprocessors can consume upwards of 40-100 watts. A typical thermal resistance requirement for a TIM being used to package ASIC devices is less than 0.5 cm2K/W, while the microprocessor TIM requirement is generally less that 0.12cm2K/W. Lower thermal resistance is desirable, since it means more heat can be removed from the device. In order to meet these thermal resistance targets conductive filler such as silver is used to provide thermal performance.